Electrical conductor



APl'il 1, 1952 P. H. sANBoRN 2,590,894

ELECTRICAL CONDUCTOR Filed Sept. 20, 1949 3 Sheets-Sheet l CLA Y INVENTOR PERCENT 12oz ATTORNEYS April 1, 1952 P. H. s ANBoRN ELECTRICAL CONDUCTOR 3 Sheets-Sheet 2 Filed Sept. 20, 1949 "2 BY M54 W ATTORNEYS- P. H. sANBoRN 2,590,894

ELECTRICAL CONDUCTOR 3 Sheets-Sheei 3 O j l 8o Filed Sept. 20, 1949 l l INVENTOR`v Jig@ .1W/fg .ff

ATTORNEYS Patented Apr. 1, 1952 AUNITED STATES PATENT OFFICE-y' ELECTRICAL CONDUCTOR Paul H. Sanborn, Parkersburg, W. Va. Application September 20, 1949, Serial No. 116,672

12 Claims. 1

This invention relates to electrical conductors, and more particularly, to ceramic oxide compositions containing oxides of iron and titanium and having electrical conducting properties.

The products of the present invention have high negative temperature coeicients of resistance and are, therefore, capable of being employed for a wide variety of applications in which ordinary resistance or conducting materials are not adapted. As disclosed in my copending application, Serial No. 116,671, led of even date herewith, similar products may be produced in the form of conducting coatings which may be applied to insulator bodies. Such conducting coatings include glass-like compositions having high mechanical strength. When applied to portions of electrical porcelain insulators employed on high voltage alternating current transmission lines so as to contact the conductor and, preferably also, the supporting member of the insulator, the conducting coatings or glazes reduce the discharges causing radio interference. The glass-like glazes may have mechanical properties, surface gloss and moisture imperviousness equel to ordinary insulating glazes conventionally7 employed on electrical porcelain insulators.

Itis known that a number of metal oxides have conducting properties which vary according to the degree of oxidation or oxygen content. In general, the lower the degree of oxidation, the greater the conductivity. At the higher states of oxidation, for example, states of oxidation approaching those corresponding to ferrie oxide or titanium dioxide, conduction is negligible. It is with the greatest difculty that uniform conducting materials can be produced utilizing oxides in lower states of oxidation such that their conductivity is determined by their oxygen content. Such oxides are usually molded and heated to produce a dense mass. The degree of oxidation is determined by the oven or furnace atmosphere surrounding the oxides. The atmosphere during firing must be reducing or oxidizing to the exact degree required to produce the specific resistivity desired. Slight variations produce great changes in specific resistivity. In addition to the difficulty in maintaining the proper furnace atmosphere, the ring temperature and the time of heating also produce variable results. As the temperature is increased the mass becomes more dense so that the furnace gases can no longer penetrate it and the reactions occur at the exposed surfaces. The final products do not exhibit stable electrical characteristics because their oxygen content is not that which exists at the most stable state of the oxides and during operation in electrical circuits, there is a trend toward higher values of resistance, particularly if the current is suiiicient to produce appreciable heating. Furthermore, most oxides produce rather high values of specific resistivity.

It is known that either ferrie oxide or titanium dioxide alone when heated under conditions in which oxygen is present during the entire heating period, produce materials of very high resistivity, i. e., negligible conductivity. In accordance with the present invention, it has been discovered that, if these two oxides are mixed in the proper proportions and red in an oxidizing atmosphere, materials are produced which have conductivities which are millions of times greater than the conductivities of either of these oxides red alone under the same conditions. The oxidizing atmosphere is that prevailing in commercial ceramic kilns in which free oxygen in excess of that necessary for the combustion of .the fuel is present.

It has been determined from an investigation of the ferrie oxide-titanium dioxide system that the composition of 2.5% titanium dioxide and 97.5% ferric oxide produces the lowest resistivity of any combination of these oxides. A specic resistivity of 1.5 ohm centimeters has been obtained with this composition prepared by pressingin steel dies and firing to 2300 F. on a 72 hour total cycle, whereas a chemically pure ferric oxide composition or a 100% chemically pure titanium dioxide composition prepared in the same manner has a specific resistivity of millions of megohms. Within the range between 100% ferrie oxide and 97.5% ferric oxide and 2.5 titanium dioxide there is an extremely rapid decrease in specific resistivity. There is tremendous reduction in resistivity between 100% ferrie oxide and the composition 99.5% ferric oxide and 0.5% titanium dioxide. As the titanium dioxide is increased above 2.5% the specific resistivity increased slowly then rapidly until the specic resistivity of the composition 97.5% titanium .dioxide-2.5% ferrie oxide is 1014 ohm per cubic centimeter.

To explain the formation of semi-conductor of low specific resistivity from two oxides which normally have extremely high specic resistivity, it is advisable to consider the atomic structure and the behavior of electrons in accordance with the modern theory of solids. Atoms of Solids are bound together and form a crystal lattice. Each atom nucleus is surrounded by a number of electrons which have definite paths and energies.

aeodsoe free electron in each level of a band'. The allowedv bands filled and empty' are separated by ranges of energy containing no levels and are known as forbidden bands because they are not permitted to electrons. If the forbidden band is Wide, the band structure is that of an insulator. If,- how-` ever, this band is small and the gap in energy be.- tween the highest lled band and the lowest empty band is of the order ot lfractional.electron volts, the material is a semi-conductor. Filled and empty bands separated by small forbidden regions is the band structure of an intrinsic semiconductor. Conduction in intrinsic semiconductors is'by excited electrons jumping intor the empty'bands.'

Extrinsic semi-conductors have'band structures similar to that of insulators except that the forbidden region contains extra energy levels-as the result of lattice imperfections'or the presence of impurities. These extra energy levels in the forbidden region act either as donators or acceptors. Atnite temperatures the donator level may pass o'nits electron to the empty band and in the process' causes conduction. Similarly, an acceptor may 'accept an electron from thelled band. The loss'of the' electron leaves anv empty level in the formerly. filled band. Conductivity may result f from alien/electrons in a normally empty band ora fewvaca'ncies in a normally nlled ba-nd.` It is`"'convenient' to consider the vacant levels as holesin' a nearly full band. These holes act as positive charged electrons.

Mostl extrinsic semi-conductors become Aintrinsic semiconductors at elevated temperatures. Heat excites the electrons in intrinsic conduction. Extrinsic semi-conductors are most useful becausethey can be produced with low resistance at ordinary temperatures. Extrinsic semi-con ductors" are of two types:

(l) N type semi-conductors which conduct principally by electrons in a nearly empty band;

(2) P type semi-conductors which conduct y principally by holes in a nearly filled band.

The introduction of extra discrete energy levelsY inthe forbidden region account for the conductivity and determine the type of semi-conductor Whether N or P type. The titanium dioxide'- ferric oxide semi-conductor contains titanium dioxide as the impurity. It is the titanium dioxide which causes the distortion of the ferrie oxide lattice and produces changes' inthe inherent physical structure and the electrical and mechanical forces. The distortion produces the levels between the lled and empty bands.

N type semi-conductors have characteristics which diier from those of thefP. type. Ifr a metal connection is made to each end of a rod of the semi-conductor material to produce a thermocouple and if one junction is heated, the polarity of the junctions is characteristic of the type of semi-conductor. If the lead from the hot junction is positive and the lead from the cold junction is negative, the semi-conductor is of the Nf type. Should the hot junction be negative and the cold junction be positive.l the semi-conductorfis ofV the "P type'.

A ferrie oxide-titanium dioxide semi-conductor as an element of a thermocouple with copper, platinum, gold and other metals as the metallic element has positive polarity at the het junction and therefore exhibits the characteristics of an "N type semi-conductor.

The point contact reotication properties of a semconductor are also an indication of the type of semi-conductor. If the current flow is greater Whenthecat Whisker or point .contact is positive than when negative, the semi-conductor is of the N type. If greater current flows when the point contact is negative, the senat-conductor is of the P type.

Als-a pointcontact rectier with a ferrie oxidetitanium. dioxide semi-conductor, the greatest current'flow is with the point contact made positive.'V The ferric oxide-titanium dioxide semiconductor, therefore, is of the N type.

Since the conductivity of a semi-conductor is the result of the introduction of extra energy levels in the forbidden zone as the result of lattice imperfections, impuritiesor the effect of the impurities inproducing lattice imperfections, X-ray diiraction patterns were made oithe ferrie oxide-titanium dioxide semi-conductor materials and Vof pure ferricoxide to observe the changes. Finely vground crystal powder produces a diffraction pattern of sharp lines,` the position andA intensity'of which are completely'cha'ract'eristic for each pure' single compound. Whena' crystalline substance is exposeottov a fine beam of monochromatic Xlradiation', a series of secondary reiiected beams emerge which have deiinite spacings and intensities` depending upon the atomicl structure of the substance irradiated. Thesebeams are recorded on a photographiclm. The X-ray i'eection'sfrom crystalline planesare criticaland emerge atspecic anglesonly. The reason'lfor this is that the crystal is'anorderly arrangement of particles in a three-dimensional lattice. The actualdistance between planes may be determined from the'X-ray diffraction pattern; The interplanar distancesy calculated from the X-ray diffraction pattern have been determined for thousands of chemical compounds' and minerals. These data lhave been indexed in a card le system data cards for'thel identification of crystalline materials, available from the American Society for Testing Materials. These cards' contain the interplanar distance (d) in angstron units. Determinations have been made upon selected compositions as follows:

Sample l- Sample 2- Sample 3- 01 97.5% Ferrie Oxide, 66.6% Ferrie Oxido. Igrlclkgdc 2.5% Titanium ni- 33.4% Titanium Dt oxido oxide 2, 69 2. 635 1 Al. 98 2. 5l 2. 475 1 2li 2. 21 2. 16 2. 7G 1.84 1. 818 l 2. 46 1.72 1.673 2. 23 l. 475 1.46.()` 1.985 1.440 1.872 1. 300 1. 735y l l. G55l I Lines of titanium dioxide.

The samples were fired in the same kiln and at the same time at 2300 F. on a 72 hour schedule. The spacings (d) of the sample composed of 100% C. P. ferrie oxide are identical With the ASTM card index for ferrie oxide. It will be observed from the spacings of sample No. 2 that they are similar to those present in sample No. 1 with the exception that sample No. 2 has smaller lattice spacing, indicating a compression of the lattice. The conclusion is that the 21/% of titanium dioxide produces a compression of ferrie oxide lattice or lattice imperfection. Titanium dioxide plays the role of an impurity. Sample No. 3 containing a larger amount of titanium dioxide shows an expansion of the lattice of ferrie oxide. The spacings are compressed with 21/2% titanium dioxide and larger amounts produce an expansion of the lattice.

The distortion of the ferrie oxide lattice by the titanium dioxide occurs even if large amounts of other ceramic materials are employed. For example, acomposition containing clay as well as ferrie oxide and titanium dioxide was prepared and ired under the same conditions as the above samples.

Sample 4- 50% Ferrie oxide. Titanium dioxide,

40% Ball clay ese so@ l Lines of ferrie oxide.

By comparison with Example l it will be observed that the lattice spacings of the ferrie oxide show a slight expansion.

The electrical conductors of the present invention therefore comprise fired mixtures of ferrie oxide and titanium dioxide. In general, the materials having the lowest resistivity are those which contain only pure ferrie oxide and titanium dioxide and have a small amount of the titanium dioxide relative to the ferrie oxide. For example, in compositions containing only ferric oxide and titanium dioxide, the lowest resistivity occurs at about 2.5% titanium dioxide. Decreasing the percentage of titanium dioxide causes the resistivity to increase and after the amount of titanium dioxide has been decreased to approximately 0.05%, further decrease in the titanium dioxide causes the resistivity to increase very rapidly. Also, increasing the amount of titanium dioxide causes the resistivity to gradually increase until relatively large percentages of titanium dioxide `are reached, after which a still further increase of titanium dioxide causes a large increase in resistivity.

Substantially any other ceramic materials, i. e., any other oxides or mixtures of oxides ordinarily employed in making ceramic materials may be added to the ferric oxide-titanium dioxide coinpositions just discussed. Such ceramic materials ma be, for example, various silicates such as aluminum silicate as well as silica, alumina, zirconia, and various fluxes such as alkali metal or alkaline earth metal oxides, zinc oxide, borax, boric acid, etc. The effect of adding such materials is primarily that of adding a diluent. The resistivity of the composition at rst gradually increases as the amount of other ceramic materials is increased. As greater amounts of such materials are added to the composition, however, the rate of increase of resistivity becomes greater and a point is reached at which the resistivity rapidly increases. The point at which this occurs depends upon the ratio of titanium dioxide and ferrie oxide, it being possible to addgreater amounts of the other ceramic materials when the ratio of titanium dioxide to ferric oxide is low, i. e., when the percent of titanium dioxide based upon the total amount of ferrie oxide-titanium dioxide present is between approximately 1% and 10%. Various admixtures of ceramic materials can be employed in order to improve the physical characteristics of the conducting material. That is to say, mixtures of pure ferric oxide and titanium dioxide, when red, may not have the desired structural strength but by adding silicates or other of the known ceramic materials in accordance with well known ceramic vformulas. resulting products having the desired physical properties can be produced.

All of the conducting materials of the present invention have a rather high negative temperature coeiiicient of resistance. Those containing pure ferric oxide and titanium dioxide as well as those containing ceramic materials such as silicates, silica, light metal oxides, etc., all have substantially the same negative temperature coeicient of resistance. Compositions of this nature will have their conductivity increased approximately 10 times when the temperature is increased from 32 F. to 212 F., the negative temperature coeflicient of resistance increasing as the temperature is increased, and decreasing as the temperature is decreased.

The addition of heavy metal oxides added to either the conducting materials containing pure ferric oxide and titanium dioxide or to compositions containing other ceramic oxides will ordinarily V`change the temperature coeicient of resistance. For example, the addition of small amounts of copper oxide will increase the negative temperature coefficient of resistance while the addition of small amounts of chromium oxide will decrease the negative temperature coeicient of resistance. Other heavy metal oxides, for example, oxides of vanadium, manganese, cobalt and nickel as well as those of tungsten, molybdenum and bismuth are also useful in compositions of the present invention, for example, oxides of the latter two metals are useful for reducing the fusion point of the compositions Without using the basic oxides previously mentioned although' they as well as the other heavy metal oxides may also be employed in conjunction with the basic oxides or other fluxes discussed above. Low resistance, low fusibility silica-free compositions may be produced by using molybdenum or bismuth oxides or both. In general, the amount of heavy metal oxides other than ferrie oxides and titanium dioxide incorporated into the conductor composition will not exceed the amount of titanium dioxide present, or in any event, will not exceed approximately of the ferrie oxide. Although certain of the heavy metal oxides. for example copper oxide, are P type semi-conductors, their employment in ferrie oxide-titanium dioxide semi-conductors inv the amounts mentioned does not alter the N type conducting characteristics.

It is therefore an object of the present invention to provide an improved electrical conducting material having any desired resistivity, including a low resistivity, in which the conducting materials comprise a vitriied mixture of oxides which separately are insulating materials.'

Another object of the present invention is to provide an improved ceramic electrical conducting material. having any desired resistivity incl'u'ding aflo'w resistivity and. alsol having a negative' temperaturelcoeicient of resistance.

Another object of the invention is toV provide an improved-resistor element having a high negative temperature coemcient of resistance.

A further object of the inventionV is to provide a' process-of making acera-mic" electrical con ducting material* having ahigh negative temperature coeic'ient of resistance.

Other objects and advantages of the invention will appear. from the following description of preferred embodiments thereof given in connection with: the attached drawing, of which:

Fig. 1y is a triaxial diagram showing compositions suitable-for conducting materials including resistor elements;

Fig. 2 is a graph showing theresistivlty of-iired compositions made up of pure'tita-nium dioxide andv ferrie oxide for small percentages of titanium dioxide;

Fig'. 3 is a schematic diagramy showing a simple temperature measuringsystem including a re sistor element in accordance with the present invention;

Fig. 4 is a schematic diagram showing a re sistor element of the present invention for controlling'the frequency of modulation of a radio frequency wave for temperature indication at a distance;

Fig., 5` isa schematic diagram illustrating the employment of a resistor element in accordance with the present invention for temperature control of relays;

Fig. 6 is a schematic diagram illustrating voltagel regulation by a resistor in accordance with the present invention; Y

Fig. 7 is a view similar to Fig. 6 showing a diiforent type vof voltage regulator;

Fig. 8 is a schematic diagram showing a motor starting arrangement using a resistor in accordance with the present invention;

Fig. 9 is a diagrammatic section of a lightning arrester showing the employment of the resistivity of the material of the present invention therein;

Fig. l is a view similar to Fig. 9 showing a modified lightning arrester; and

Fig. l1 is a curve showing the relation between time, voltage and current in a typical resistor heated by current flowing therethrough.

For purposes of illustrating the present invention, the resistivity characteristics' of various compositions including three components, namely, titanium' dioxide, ferrie oxide and clay, have been shown in the form. of a triaxial.y dia gram in Fig. l. Substantially pure titanium dioxdeand ferrie: oxide Were employed but the clay was a mixture. of ball clay and bentonite, bothl of which contain substantial amounts of materials other than aluminum silicates. In all compositions containingy clay, of the total composition was bentonite, which is an extremely plastic material and was used to give good ceramic properties to the compositions having low clay content, and the remainder of the clay in each composition was ball clay. The particular ball clay employed contained 1.6% titaniuin dioxide, and 1.5% ferrie oxide as impurities andthel compositions of the triaxial diagram are corrected to show the actual amount oi titanium dioxide and ferrie oxide therein. This, in conjunction with improved technique in making electrical contact with thel fired compositions,v results` in general, in lower resistivities for a given composition than those` reported in my copending application Serial No.` 408,400, lfiled August 26, 1941', now' abandoned. InI all cases', the compositions were iired under the same conditions, i. e., at 2300"' F. on a '72 hour total cycle including heating vand cooling. The resistivities are given in ohms per centimeter cube at '70 F.. i. e., the speciiic resistivity at F.

In Fig. i, the point 5' indicates a composition containing no clay but containing 2.5% titanium dioxide and 97.5% ferrie oxide. This composition provided a resistance of approximately 1.5 ohms percentimeter cube which is approximately the lowest resistance which can be obtained employing titanium dioxide and ferrie oxide. Any substantial change in this composition increases the resistivity. in general, the lowest resistivities are obtained by employing a temperature during ring which is just suflicient to vitrify and an extended time of treatment at this temperaturc so as to insure a uniform composition throughout the sample. A firing temperature lower than 2300 F. would produce a series with higher individual resistivities while a higher temperature would produce, in general, a lower resistivity for each of the samples containing clay, providing over-firing with resulting devitrication does not occur. The compositions containing titanium dioxide and ferrie oxide without the addition of clay may have organic binding agents, such as waxes or similar bonding materials added thereto to improve the working properties but such bonding materials are burned out during firing and have no appreciable eiect upon the iinal resistivity. however, all of the compositions of the triaxial diagram were produced by merely pressing dampened powder made up of the mixed ferrie and titanium oxides and clay in steel dies prior to firing.

After reaching the maximum firing temperature the rate of cooling affects the i'inal resistivity. if the vcooling is rapid, the resistivity is lower than if the rate of cooling is slow.

The red samples were provided with terminals by application of metal compounds of the type used for ceramic decoration. Com-'- mercial gold was employed in producing the samples, but platinum and silver as well as gold have also been used and have proven satisfactory pro viding they are red to the proper temperature since these materials usually contain a fusible portion which tends to form a high resistance layer on the semi-conductor if the ring temn perature is too high. It will, of course, be understood that the ceramic compositions are rst fired to the proper temperature, after which the ceramic metal is applied to provide terminals, and then the conducting elements again red at lower temperature. The rate of cooling following the metallizing operation has an effect upon the final resistivity in somewhat the same manner as the cooling following the original firing of the conducting bodies. The resistivity will be slightly lower if the rate of cooling is rapid. The reason for this effect is apparent when it is realized that the semi-'conductor compositions have high negative Atemperature coefficients o1 resistance. Heating decreases thD resistance while cooling increases the resistance. If the semi-conductor material is heated and cooled slowly it will have a value oi resistivity corresponding to the final temperature. If, however, it is cooled rapidly, molecular rearrangement cannot take place rapidly enough and some of the characteristics' imparted by higher temperatures are retained. It is. not

dicult to produce successive batches having almost identical characteristics when the materials are subjected to similar conditions in the processing, particularly the temperature of iiring and of the cooling cycles following ring and metallizing.

Referring again to Fig. l, the useful ranges of compositions are approximately bounded by the lines joining the points 6, 'I and 3. These points are as follows:

(6)'-titanium dioxide 0.05%, ierric oxide 99.95%,

clay 0.

(7)-titanium dioxide 1.12%, ferrie oxide 23.88%,

clay 75%.

(iD-titanium dioxide 63%, ferrie oxide 37%, clay 0.

` The line 9 connecting the points 6 and 'I is drawn approximately through the points at which a further decrease in the titanium dioxide content, i. e., a decrease in the ratio of titanium dioxide to ferric oxide will cause an extremely large increase in resistivity. This is illustrated in Fig. 2 in which the curve I I represents resistivity in ohms per centimeter cube plotted against a small percentage of titanium dioxide, the remainder of the composition 'being ferric oxide. The resistivity was measured on samples iired at 2300 F. on a 72 hour schedule with ceramic gold terminals applied thereto as discussed above. A minimum point of resistivity was obtained at 2.5% titanium dioxide and the curve becomes substantially vertical at a composition contain ing 0.05% titanium dioxide. Similar curves can be drawn for each percentage of clay and their form is similar to that of Fig. 1. The percentage of titanium dioxide in the total composition which gives minimum resistivity will increase vas the percentage of clay is increased and the ratio of titanium dioxide to ferric oxide which gives minimum resistivity will also increase. Similarly,

the percentage of titanium dioxide in the tota-l composition, at which the resistivity curve becomes substantially vertical for decreasing amounts of titanium dioxide, will increase, but as stated above, the shape of the curve for each percentage of clay will be very similar to that oi Fig. 2. Points I2, I3 and I4, which are close to the point at which the resistivity begins to increase rapidly, have also been plotted on the triaxial diagram and the straight line 9 has been drawn therethrough. It will be noted that this line is close to the zero titanium dioxide axis of the triaxial diagram.

In a similar manner, straight line I6 has been drawn to connect points 'I and 8 on the triaxial diagram. This line passes through the points which closely approximate those at which the resistivity of the compositions rapidly increases as the percentage of titanium dioxide is increased, i. e., the ratio of titanium dioxide to iron is increased while the amount of clay is maintained constant. This can be seen qualitatively from the triaxial diagram, as in general, the points to the left of the line I6 indicate very high resistance as compared With the points at the right of the line I6. That is to say, curves showing resistivity plotted as ordinates against percentages of titanium dioxide in the total composition as abscissa when the amount of clay is maintained constant, become substantially vertical at points adjacent the line I6.

In compositions for producing conducting elements usable as negative temperature coefficient resistors, etc., the amount of titanium dioxide in the total composition ranges from approximately 0.05% to 63 the amount of ferric oxide ranges from approximately 23.88% to 99.95% and the amount of other ceramic oxides, such as those contained in clay, ranges from approximately zero to 75%. In addition to never being greater than 63%, the percentage of titanium di-v oxide in the ceramic compositions should not be greater than 4.7165 vtimes the percentage of ferric oxide minus 111.51% and in addition to never being less than 0.05%, the percentage of titanium dioxide should not be less than 1.4560% minus 0.01407 times the percentage of ferrie oxide. Also, in addition to never being greater than 75% of the composition, the percentage of other ceramic oxides, such as those contained in clay, should not be greater than 98.54470 minus 0.9859 times the percentage of ferric oxide.

rihe above limits completely denne the area on the triaxial diagram bounded by the lines 9 and I0 and the axis of the diagram for zero amount of clay. The compositions included within the limits above expressed will all have a specic resistivity less than approximatley 30,000 ohms per centimeter cube. A small increase in the ratio of titanium dioxide to ferric oxide near the line I5 will cause a rapid increase in specific resistivity and the same is true of a small increase in the ratio of clay to the total amount of titanium .dioxide and ferrie oxide. Similarly, a small decrease in the ratio of titanium dioxide to ferric oxide near the line 9 will cause a large increase in specific resistivity.

The resistivities reported on the triaxial diagram of Fig. 2 are approximately the minimum resistivities which applicant has been able to obtain using the best techniques known to him. Variations in the firing procedure including temperature and heating and cooling periods, may be employed to somewhat modify the resistivities, but in any event, the boundaries represented by lines 9 and it on the triaxial diagram pass very closely to the points at which variations in the composition cause a rapid increase in resistivity. This is true irrespective of whether the other ceramic oxides are the clay (aluminum silicate) of the triaxial diagram or other ceramic oxides such as silica itself, zirconium oxide, potassium oxide, sodium oxide, etc., or Whether other heavy metal oxides such as chromium, copper, cobalt, manganese, etc., are present. An example of how changing the type' of ceramic material other than the active oxides will vary resistivities of the composition even though the total amount of such materials is maintained constant and the amount and types of active oxides are maintained constant, the following table is given:

Samples- No. l No. 2 No.3 No. 4

Titanium dioxide 10 l0 10 Ferrie oxide The titanium dioxide-ferrie oxide semi-conductors have negative temperature coefficients which increase at lower temperatures and decrease at higher temperatures. Most of these semi-conductors containing oxides and silicates as diluents show 2% to 3% change in resistivity per degree C. at 25 C. Some heavy metal oxides as diluents product variations from these values. Compositions containing copper oxide, which reduces the firing temperature if present in appreciable amounts, increases the change per degree while chromium oxide decreases the change per degree. While copper oxide is a P" type semiconductor its use in a titanium dioxide-ferrie oxide does not alter the N type characteristics.

Other materials than the oxides may, of course, be employed in producing the original composition so long as these materials are converted to the oxides during the firing operation. Such other materials are, for example, hydroxides and carbonates as well as the silicates of magnesium, aluminum, zirconium, etc. In this connection, the silicates are considered to be oxides. In addition, a wide variety of fluxes, such as calcium oxide, sodium or potassium oxides or compounds of silicates of alumina with sodium, potassium or calcium, such as feldspar, may be employed as well as other light metal oxides such as zinc oxide. In fact, substantially all of the materials conventionally used in ceramics, either to form the body of the fired article or as a flux for lowering its melting point may be employed in the titanium dioxide-ferrie oxide compositions of the present invention.

The heavy metals employed in combination with titanium in accordance with the present invention are preferably added in the oxide form at their highest degree of oxidation or at least in a state of oxidation greater than the lowest. However, any compounds of these metals which are converted into the oxide during firing under oxidizing conditions may be employed. The same is true of the light metal or non-metal oxides employed as part of the composition. Thus in general, the carbonates are suitable and in some instances, the halides. For example, with iron any of the oxides can be employed if care is taken to oxidize the same during ring. Also the sulfates and chlorides may be employed as well as ferrous ammonium sulfate. With titanium the other oxides such as T1203 and also titanium sulfate may be employed instead of titanium dixode and naturally occurring titanium ores such as rutile can also be employed. It is apparent that the hydroxides of any of the metals may likewise be employed as the Water of crystallization is driven off during firing. Furthermore, natural ores containing iron or other heavy metal oxides and titanium may be employed with or without suitable additions of oxides to produce the desired ratio of titanium to other heavy metal oxide. Thus ilmenite which is a mixture of iron and titanium oxide, or joaquinite which is an iron titanium silicate, or titanite which is a calcium titanium silicate, may be employed. In general where the titanium is added in the form of an ore, for example, in combination with iron or silicon or other metals, the ratio of titanium to iron or other heavy metal oxide must be increased somewhat over those necessary with the individual oxide as the effectiveness of the titanium appears to depend somewhat upon its previous history. To illustrate this factor the following l2 composition was prepared, changing only the material containing the titanium dioxide:

The following table shows the variations in. resistance with the source of titanium dioxide:

Resistance ohms per cmi. at 70 I".

Source It will be apparent that anatase gives the lowest resistivity and that ilmenite in which the titanium occurs with iron` gives the highest resistance.

Resistance units in accordance with the present invention may be prepared by substantially any of the methods employed in producing ceramic bodies. The direct method of merels7 grinding materials together in a pebble or ball mill with sufficient water to make a slurry is preferred. After thorough mixing and grinding, the excess water may be removed by iilter pressing or drying in the air or on plaster slabs. The resulting plastic mass may be pressed in dies or may be extruded into rods or tubes through proper sized dies commonly used in the ceramic industry. If plastic clays are not used, it is desirable to use organic binding agents known to the art, such as various waxes, dextrin, gum arabic and similar substances. The rods may then be dried and fired either before or after cutting into suitable lengths for resistance units. Alternatively, the resistor units may be dry pressed in which case the slurry from the pebble mill is dried to a lower moisture content than that necessary for extrusion. In proper condition the material will be a damp crumbly mass and in this state the material may be placed in steel or rubber molds and molded by pressure. It will be apparent that any of the usual pottery formingy methods such as dry-pressing, extruding, casting, hot-pressing, or jiggering may be employed. It is possible also to produce conducting grains suitable for employment as resistance material in lightning arrestors inwhich case the mixture from the ball mill is substantially completely dried and the resulting hard material crushed and screened to obtain the proper grain size. The grains are then fired in a suitable container such as a fire clay Crucible to the proper temperature.

Itis also possible to re a powdered mixture of titanium and iron oxides or such a mixture containing other materials as one or more of the other heavy metal oxides mentioned, which produces a composition having a rather high fusion point, to the proper temperature to produce conductivity and then pulverize the resulting mix. This powdered conducting material could be then mixed with low temperature ceramic uxes and fired again at low temperatures. This produces conducting articles at low rlring temperatures. Such uxes may be lead oxides, kryolith, borax, boric acid, and other materials now used in the production of porcelain enamel.

. The correct temperature for firing any of the compositions above discussed is that which will cause vitriiication of the material without substantial devitriflcation during the time which the compositions are subjected to such temperature. Best results are ordinarily obtained by employing the longest iiring cycle which is commercially feasible and the lowest firing temperatures which will cause vitrilcation under such conditions. Somewhat higher temperatures can, however, be employed with a correspondingly shortened iiring cycle. Mixtures of substantially pure ferrie oxide and titanium dioxide will usually be fired at a temperature in the neighborhood of 2300 F. but with compositions containing substantial amounts of other ceramic materials, the nature of and proportions of such other materials may be varied as discussed above so as to secure ring temperatures ranging between approximately 1500 and 2500 F. while still producing iired compositions having relatively low specific resistivities. That is to say, substantially any known or suitable ceramic materials or mixtures of ceramic materials which can be molded and then fired to produce bodies of adequate mechanical strength can be employed instead of the clay shown for illustrative purposes in the triaxial diagram. Such ceramic compositions will, in general, produce iired products of extremely high speciiic resistivity in the absence of both titanium dioxide and ferrie oxide when iired under oxidizing conditions producing a state of oxidation equivalent to ferrie oxide or titanium dioxide but when admixed with ferric oxide and titanium dioxide in the proportions given as to clay on the triaxial diagram and red under such oxidizing conditions, specic resistivities of the same order as those shown are obtained. Although mixtures of substantially pure ferric oxide and titanium dioxide produce fired bodies of relatively low specic resistivity useful for many purposes, resistors in accordance with the present invention will, in general, contain at least of other ceramic materials to either provide increased Workability of the composition before firing or increased mechanical strength after firing. By employing glass-forming materials, such as silica and a iiux for the other ceramic materials, a glass or glass-like material having a relatively low specific resistivity can be produced upon firing. In such glasses the amount of other ceramic materials will ordinarily be at least 40%.

Various methods may be employed for insuring eiiective connection between the ends of the resistor units and resistor leads. For resistor elements having a low enough resistance to carry an electroplating current, conducting metals such as copper, nickel, tin, chromium, cadmium, etc., may be electrically plated upon the ends of the resistor unit. For all resistors including high resistance units metal may be sprayed upon the ends of the resistor units by any of the metal spraying processes known to the art. Another method particularly useful for precision resistors isI to coat the ends of the resistor elements with ceramic liquid gold, platinum or other noble metal such as is used to decorate dishes. This material may be painted on the ends of the resistors or the resistors dipped thereinto and allowed to dry. By heating to approximately 1500 F., in the case of platinum, the platinum is reduced to metallic form and may then, if desired, be coated or plated with a conducting metal such as copper or silver. Resistor leads may then be soldered or clamped against the conducing metal on the ends of the resistor units and if desired the entire resistor unit including the ends of the leads adjacent the resistance material may be imbedded in insulating material such as thermoplastic or thermosetting synthetic resins, for example, bakelite. If the leads and conducting metals on the end of the resistors have a suiiiciently high melting point the entire resistor element including the ends of the leads adjacent the resistor may be dipped in a low melting point non-conducting glaze and again red to provide resistors completely encased with glaze or glasslike insulating material.

A simple temperature measuring circuit is shown in Fig. 3 in which a resistor 32 is connected in series with a battery 33 and a current measuring instrument 34. The current measuringv instrument 34 is preferably a micro-ammeter or milliammeter so as to operate with a low current in order that the current flowing through the resistor 32 has no appreciable heating effect upon the resistor. By applying a constant voltage across the resistor and meter 34 by means of a battery 33 or similar source of electromotive force the deflection of the meter 3:3 can be made a substantial straight line function of temperature over a relatively wide range and the meter may be calibrated directly in temperature units. By employing a resistor 32 which has a resistance large in comparison with the resistance of the rest of the circuit, the resistor may be located at a distance :from the meter 34 to give remote temperature indications. For example, such a system can be employed for measuring temperatures at the tops of mountain peak-s with the meter positioned at the base thereof. Such an arrangement may also be employed for such temperature indications as the water or oil temperature of motors in automobiles and airplanes. It wil be apparent that the meter 34 can be a recording as well as an indicating instrument and that other factors such as the voltage drop across the resistor 32 when it is placed in series with another resistor or the volt age drop across such other resistor to indicate temperature Another type o temperature measuring or indicating system is shown in Fig. 4 in which a resistor 35 of the type heretofore descibed is employed to vary the frequency generated by a tube 3E connected as an oscillator' which frequency in turn modulates the output of a radio frequency oscillator 31 to provide a radio frequency voltage modulated with a lower frequency which varies with the temperature cf the resistor 35. The tube 36 is shown connected as a relaxation oscillator but it is apparent that other types of oscillator circuits whose frequency can be made a function of resistance can be employed. The radio frequency from such a system as that shown in Fig. 4 may be radiated by an antenna 33 to form a transmitter. Such a transmitter ma-y be carried by a free ballon employed in meteorography or may be positioned at remote points upon the ground and the signal picked up by a suitable receiving system by which the signal is demodula'ted and applied to a frequency meter or recorder which may be calibrated in temperature units.

As shown in Fig. 5 a resistor 40 having a high negative temperature coeiicient of resistance may be employed for temperature control of relays, for

example, a resistor 48 may be connected in series with a source of voltage 4|, a relay coil 42, and if desired, a current measuring instrument 43 calibrated in temperature units. Upon increase in temperature the current through the resistor 40 and relay coil 42 increases until the relay closes. Adjustment ofthe closing temperature `may be inadefby varying the lvolta'geli or theresistancev oa variableresistor ll'in parallel withthetemperature'responsive resistor 55.' Itisapparent that the resistor-244' could beplaoed inserieswith thefresistorrdf.- and that for relay closing upon a decrease (if-temperature the resistor 45 could be placed fin parallel with the relayt.. The temperatureresponsive resistor of the present' invention may' also bey employed for voltage regulation. for either- D. C. or A. C. currents in: which case the thermo-responsive resistor 451 (Fig. 6) maybe placed in series with the source 47 and a ballast or` limiting resistorll which is preferably of a metallictype having a positive temperature coeiilcient' of resistance; The voltage to be regulated. is shown as applied'across a load in the form of a' resistance 49; Upon increase of voltage at the source 41- thei increased'current through the resistorli increases the temperature thereof to lower the resistance of the resistor L thus increasing the current through the resistor 58 and causing greater drop across the resistor 45 to lower the voltage' across the resistor at. If the load t9 is decreased to take increased current the voltage drop across the resistor 55 increases to decrease the voltage drop across the resistor t5 thus allowing the temperature of the resistor 5.5 to decrease and increase its resistance causing the voltage drop across-the resistor 46 and the load is to increase. The opposite eiect is produced upon increase of the load i9 so that the circuit shown tends to maintain the voltage across the load i9 substantially constant.

Reference is made to Fig. ll to illustrate the relation between time and current and voltage of atypical resistor of the present invention heated by current ilowing therethrough. As shown in the circuit of this 'gure, a resistor 55 having a resistance of 38-0 ohms at 30 C. was connected in series with a wire wound resistor '5I having a resistance of ohms across a source of 110 v. The curve shows the drop in voltage across the resistor 5U after the voltage was applied and the curve F shows the-increase in current. Under these conditions the value oi resistance of the resistor 55 dropped from 380 ohms to approximately 2.5 ohms, or to about 1A@ of its original value. By varying the physical size of the resistor relative to its resistance at a given temperature, the time for a given temperature, the time for given resistance drop can be varied within wide limits. Since the resistors of the present invention can withstand high temperatures approaching their firing temperatures as` well as low `temperatures such Wide changes in resi-stance as above described can be utilized. 'f

The thermo-responsive resistor of the present invention may also be employed for voltage regulation of generators. As shown in Fig. 7 the thermo-responsive resistor 52 may be connected across all or a portion of the neld 53 of a generator 54. Ir the voltage of the generator 5ii tends to increase, current iowing through the resistor 52 increases its temperature and lowers its resistance to more eiiectively shunt the field 53 to decrease the voltage of the generator 54. By employing a resistor 52 of sufficient size to radiate effectively the heat produced by current flowing therethrough without materially increasing its temperature the resistor 52 may be employed to regulate the voltage of the generator 54 in accordance with the temperature of the atmosphere in which the resistor 52 is positioned. Thus the resistor 52 canbe employed to increase-the charging rateofanautomobile battery'as the atmospheric temperature. decreases-soesto automatically-increasethe rate Ofchargingan automobile battery; inthe winter` and-decreasethis rate insummer.

In such a system the resistor52 shouldbe located` Y atapoint remote from the engine so as tonot be subject toengine heat.

Referring to Fig. 8, the thermo-responsive resistor 55 may be'employedfor motor starting; In

thediagramshown theresistor `55 isin series with the motor 55 and has `a .relatively high resistancer at ordinary temperatures. Thus a relatively low.

voltage is initially appliedto the motory 55. Cur-y rentflow through the resistor 55 graduallyl decreases the resistance thereof, thus decreasingqthe voltageY drop acrosstheresistor and increasingvv the voltageacross the motor 5e. When the volte age across :the motor increases to a predetermined. value the relay 5l operates to short-circuittheresistor 55, at .the same time .completinga l0ck-ing circuitl for holding the-relay 5l energized during:

runningor" the motor. The resistors of the present invention may be built of relatively large sizesv capable of standing extremely high temperatures,4

that is; any temperature below 'the relativelyhighv firing temperatures employed in producing the resistor and'can be employed in accordance withl the circuit of Fig. 8 to start relatively largemotors of either the direct current or alternating current type as well as to gradually apply voltage to any other type of equipment. Itwill be apparent that conventional starting resistors may ce connected in parallel with the resistor 55 and the resistor 55 made of relatively small size and merely employed tok provide a time delay before shorting out the main resistor. A series of resistors can of course be employed with appropri-H ate time delay resistors of thepresent invention and relays for cutting out' a series of` main re,- sistors'in steps. It will also be apparent that the resistors of the present invention may be em.

ployed as heating elements even for high temperature operations particularly if they contain substantial amounts of refractory oxides such4 as.

zirconia or alumina. y

Another important application of 'the resistance material of this invention is in lightning arresters. As shown in Fig. 9, such a lightning arrester may comprise a tube 5l' of insulating material such porcelain closed across itsbottom with a plug of insulating material 58 through which a conductor 59 extends terminatingin a contact member et. 'I'hetube 5l may bepartially filled with grains of resistance material t! having a. high negative temperature coeiiicient of resistance and in the upper portion of the tube 5l a seriesof spark gaps 62 may be provided' and separatedV from the resistance grains, by an insulating barrier 53. The entire lghtningarrester may be provided with a cap of insulating material Slimade of porcelain 01 like material, with a conductor 65 extending therein and having a terminal 65 forming.. part oi' the spark gap 62, As shown in Fig. l0, discs 55 of resistance material having a high negative temperature coefficient oi' resistance may be substituted for the grains 6|. Otherwise the lightning arrester structure may be similar to that of Fig. 9.

Resistance material such as the grains (il of Fig. 9 or the discs 68 of Fig. l0 have a relatively high resistance at ordinary temperatures and are in series with the spark gaps so that a relatively high voltage must be applied between the contacts 65 and 65 before any current flows through the arresters. When surges due to lightning or other causes appear on thev power line current ows through the; arrestersl and the enf 17 Y ergy of the surges is dissipated as heat. The resistor material for lightning arresters should be capable of withstanding high temperatures and compositions including substantial amounts of such refractory oxides as zirconia or alumina in addition to titanium oxide, another oxide of a heavy metal and a plastic binder such as clay are preferred.

The resistors of the present invention have various other important applications such as compensation for frequency drift in oscillator circuits and intermediate frequency application circuits of radio apparatus due to changes in temperature, and compensation'for variations in temperature for the cold junctions of thermocouples. They may also be employed in Wheatstcne bridge resistance thermometers, re alarms, controlling the heating current in starting uorescent lamps, etc. It will thus be seen that I have provided an improved conducting or resistance material prepared from inexpensive materials which are themselves substantially non-conducting. The improved conducting material can be made of low resistance and has a high negative temperature coeiiicient of resistance. sistance and temperature coiilcient of resistance at a given temperature can be varied Within Wide limits and the new material has a Wide variety of uses.

This application is a continuation-in-part of Amy copending application Serial No. 408,400,

`99.95% ferrie oxide and to 75% other ceramic oxides by Weight, said other ceramic oxides being in a state of oxidation equivalent to that of -ferric oxide and titanium dioxide, the percentage of any heavy metal oxides other than said ferric oxide and said titanium dioxide not exceeding the percentage of said titanium dioxide, and in any said composition the percentage of titanium dioxide not being greater than that obtained by multiplying the percentage of ferric oxide by .4.7165 and then substracting 111.51%, or less than that obtained by multiplying the percentage of ferric oxide by 0.01407 and subtracting the result from 1 4560% and the percentage of 'other ceramic oxides not being greater than that lobtained by multiplying the percentage of ferric oxide by 0.9859 and subtracting the latter result from 98.544%.

1 2. An electrically conducting ceramic body .having a negative temperature coefficient of resistance, said body being a iired substantially .homogeneous vitried composition consisting of from 0.05 to 63% titanium dioxide, 23.88 Yto 99.95% ferric oxide and to 75% other ceramic oxides by weight, said other ceramic oxides beinglin a state of oxidation equivalent to that of ferrie oxide and'titanium dioxide, the percentage of any heavy metal oxides other than isaid ferrie oxide and said titanium dioxide not exceeding the percentage of said titanium dijoxide', and in any said composition the percentage of titanium dioxide not being greater than that 'obtained by multiplying the percentage of ferric oxide by'4.7165 and then subtracting 111.51

The revoxides by Weight, said other ceramic oxides including other heavy metal oxides and being in a state of oxidation equivalent to that of ferrie `oxide and titanium dioxide, the percentage of said other heavy metal oxides not being greater than the percentage of said titanium dioxide, and in any said composition the percentage of titanium dioxide not being greater thanthat obtained by multiplying the percentage of ferric oxide by 4.7165 and then subtracting 111.51%, or less than that obtained by multiplying the percentage of ferrie oxide by 0.01407 and subtracting the result from 1.4560% and the percentage of other ceramic oxides not being greater than that obtained by multiplying vthe percentage of ferrie oxide by 0.9859 and subtracting the latter result from 98.544%.

4. A resistor unit comprising an electrically conducting ceramic body having a negative temperature coeicient of resistance, said body being a red substantially homogeneous vitrified composition consisting of from 0.05 to 63% titanium dioxide, 23.88 to 99.95% ferric oxide and 0 to other ceramic oxides by Weight, said other ceramic oxides being in a state of oxidation equivalent to that of ferrie oxide and titanium dioxide, the percentage of any heavy metal oxides other than said ferric oxide and said titanium dioxide not exceeding the percentage of said titanium dioxide, and in any said composition the percentage of titanium dioxide not being greater than that obtained by multiplying the percentage of ferrie oxide by 4.7165 and then subtracting 111.51 or less than that obtained by multiplying the percentage of ferric oxide by 0.01407 and subtracting the result from 1.4560% and the percentage of other ceramic oxides not being greater than that obtained by multiplying the `percentage of ferric oxide by 0.9859 and submetallic contact elements fused to said body a 55 'spaced positions thereon.

5. A resistor unit comprising an electrically conducting ceramic body having a negative temperature coeiiicient of resistance, said body being a red substantially homogeneous vitried composition consisting of from 0.05 to 63% titanium dioxide, 23.88 to 99.95% ferrie oxide and 10 to 75% other ceramic oxides by Weight, said other ceramic oxides being in a state of oxidation equivalent to that of ferric oxide and titanium dioxide, the percentage of any heavy metal oxides other than said ferrie oxide and said titanium dioxide not exceeding the percentage of said ti- -tanium dioxide, and in any said composition the percentage of titanium dioxide not being greater 'than that obtained by multiplying the percentage of ferric oxide by 4.7165 and then subtract- -ing 111.51%, or less than that obtained by multiplying the percentage of ferric oxide by 0.01407 andsubtracting the result from 1.4560% and v'and the'percentage of other ceramic oxides not 19 being, greater than that obtained by multiplying the percentage of ferric oxidev by 0.09859 and subtracting the latter result from 98.544%,. and metallic contact elements fused to said body ai spaced' positions thereon.

6. A resistor unit comprising an electrically conducting ceramic' body having a negative temperature coefficient of resistance, said body being a fired substantially homogenous vitried composition consisting of from 0.05 to 63% titanium dioxide, 28.88 to 99.95% ferric oxide and 10 to 75% other ceramic oxides by weight, said other' ceramic oxides including other heavy metal oxides and being in a state of oxidation equivalent to that of ferric oxide and titanium dioxide, the percentage of said other' heavy metaly oxides not being greater than the percentage of said titanium dioxide, and in any said composition the percentage of titanium dioxide not being greater than that obtained by multiplying the percentage of ferric: oxide by 4.7165 and then subtracting 111.51%, or less than that obtained by multiplyingl the percentage of ferric oxide by 0.01407 and subtracting the result from 1'.4560% and the percentage of other ceramic oxides not being greater than thatV obtained by multiplying the percentage of ferric oxide by 0.9859 and subtracting the latterl result from 98.54495, and metallic contact elements fused to said body at spaced positions thereon.

7'. The' method of producing a' fired composition in the form of an electrically conducting ceramic body, which method comprises, forming a vitriabler mixture of ceramic materials consisting of at least one titanium compound man amount producing in the red composition 0.05% to 63% titanium dioxide, at least one iron compound inan amount producing in the red composition 23.88 to 99.95% ferric oxide, and other ceramic materials in anv amount producing in the red composition to 75% of the corresponding oxides having a state of oxidation equivalent to that of ferric oxide and titanium dioxide, thev amount of any heavy metalr compoundsr in saidv other ceramic materials producing' a percentage. of heavy metal oxides not greater than the percentage of titanium dioxide, the amount ofv titanium compound in any said mixture producing a percentage of titanium dioxide not greater than that obtained by multipl'ying the percentage of said ferric oxide by 4.7165. and then subtracting 111.51% and' not less.v than that.. obtained by multiplying the percentage of said ferric oxide. by 0.01407 and subtracting the result from 1.4560%, amount of' other ceramic materials in any said mixture producing a percentage of said corresponding oxides not greater thanv that obtained by multiplying the percentage of said ferric oxide by 0.9859 andl subtracting the latter result from 98.544%, all percentages being by weight Yofi the ceramic composition after ring, and firing said mixture at a. vitrifying temperature in an oxidizing atmosphere for sufficient time to produce a substantially homogeneous vitriedand the 20 compound in an amount producing' in the fired compositori 23.88 tol 99.95% ferric oxide, and other cerarmic materials in an amount produc ing in the fired composition 10 to 75% of; thek corresponding oxides having a state of oxidation equivalent to that of ferric oxide and titanium dioxide, the amount of anyA heavyv metal compounds in said other ceramic materials produc'- ing a percentage of heavy metal oxides not greater than the percentage of titanium dioxide, the amount of titanium compound in any said mixtureproducing a percentage of` titanium dioxide not greater than that obtained by multiplying the percentage of said ferric oxide by 4.7165 and then subtracting 111.51% and not lessf than obtained byy multiplyingA the percentage of said ferric oxide byv 0.01407 and subtracting thev result from 1.4'5'60%, and thel amount ofv other ceramicv materials in any said mixture producing a percentage of said corresponding oxides not greaterv than that obtained by multiplying the percentage of said ferricy oxide by 0.9859 and subtracting the latter result from 98.544%, all.

percentages being by weight of the ceramic composition after firing, and ring said mixture at a vitrifying temperature in an oxidizing atmosphere for 'suicient time to produce a sub'- stantiallyhomogeneous vitried material in which the oxides have a state of oxidation equivaient to that of ferric oxide and titanium dioxide;

` 9'. A solid; electrically conducting ceramic bodyhaving a negative temperature lcoefficient of resistance, said body being a red substantially homogeneous vitrified composition consisting of from 0.05 to 63% titanium dioxide, 23.88 to 99.95% ferric oxide and 0 to 75% otherceramic oxidesv by weight, said other ceramic oxides being in a statev of oxidation equivalent to that of ferric oxide and titanium dioxide, the amount of any heavy metal oxides other than said ferric oxide and said' titanium dioxide being not greater than the amount of said titanium dioxide,

said' composition having a specic resistivity of at least 1.5 ohmsy and not more than 30,000 ohms.

10. An electrically conducting ceramic body having a negative temperature coeilicient of re- `sistance, said body being a fired substantially homogeneous. vitried composition consisting of from 0.05 to 63 titanium dioxide, 23.88 to 99.95% ferric oxide and 10 to 75% other ceramic oxides by- Weight, said other ceramic oxidesz in'- cluding. other heavy metal oxides and being in a state of' oxidation equivalent to that of ferric oxide and titanium dioxide, the amount of said other heavy metal oxides being notgreatervv than the amount of said titanium dioxide, said. com.- position having a specic resistivity of at least 1.5 ohms and not more than 30,000 ohms..

11. A'. resistor unit comprising; an electrically conducting. ceramic body' having a negative temperature coefficient. of resistance, said body being a fired substantially homogeneous. vitrified composition consisting of from 0.05 to 63% ti:- tanium dioxide, 23.88 to 99.95% ferricY oxidel and 0 to*V 75 other` ceramic oxides by Weight, said otherv ceramic oxides. being: in a state of oxidation equivalent' toY that of ferric oxide and ti.- tanium dioxide; the. amount of any heavy metal oxides other than said' ferricv oxideand said titanium dioxide being not greater than the amount of said titanium dioxide,l said` composition having a specific resistivity of at least 1-.5 ohms and not inorey than 30,000 ohms, and metalli-c contact elements :fusedl toI said bodyd at spaced positions thereon.

12. A resistor unit comprising an electrically conducting ceramic body having a negative temperature coeicient of resistance, said body being a red substantially homogeneous vitried compositionv consisting of from 0.05 to 63% titanium dioxide, 23,88 to 99.95% ferric oxide and 10 to '75% other ceramic oxides by weight, said otherv ceramic oxides including other heavy metal oxides and being in a state of oxidation equivalent to that of ferrie oxide and titanium dioxide, the amount of said other heavy metal oxides being not greater than the amount of said titanium dioxide, said composition having a specific resistivity of at least 1.5 ohms and not more than 30,000 ohms, and metallic contact elements fused to said body at spaced positions thereon.

PAUL H. SANBORN.

22 REFERENCES CITED UNITED STATES PATENTS Number Name Date 648,518 Ochs May 1, 1900 FOREIGN PATENTS 10 Number Country Date 552,783 France 1923 586,064 Great Britain 1947 588,271 Germany 1933 

1. AN ELECTRICALLY CONDUCTING CERAMIC BODY HAVING A NEGATIVE TEMPERATURE COEFFICIENT OF RESISTANCE, SAID BODY BEING A FIRED SUBSTANTIALLY HOMOGENEOUS VITRIFIED COMPOSITION CONSISTING OF FROM 0.05 TO 63% TITANIUM DIOXIDE, 23.88 TO 99.95% FERRIC OXIDE AND 0 TO 75% OTHER CERAMIC OXIDES BY WEIGHT, SAID OTHER CERAMIC OXIDES BEING IN A STATE OF OXIDATION EQUIVALENT TO THAT OF FERRIC OXIDE AND TITANIUM DIOXIDE, THE PERCENTAGE OF ANY HEAVY METAL OXIDES OTHER THAN SAID FERRIC OXIDE AND SAID TITANIUM DIOXIDE NOT EXCEEDING THE PERCENTAGE OF SAID TITANIUM DOIXIDE, AND IN ANY SAID COMPOSITION THE PERCENTAGE OF TITANIUM DIOXIDE NOT BEING GREATER THAN THAT OBTAINED BY MULTIPLYING THE PERCENTAGE OF FERRIC OXIDE BY 4.7165 AND THEN SUBSTRACTING 111.51%. OR LESS THAN THAT OBTAINED BY MULTIPLYING THE PERCENTAGE OF FERRIC OXIDE BY 0.01407 AND SUBTRACTING THE RESULT FROM 1.4560% AND THE PERCENTAGE OF OTHER CERAMIC OXIDES NOT BEING GREATER THAN THAT OBTAINED BY MULTIPLYING THE PERCENTAGE OF FERRIC OXIDE BY 0.9859 AND SUBTRACTING THE LATTER RESULT FROM 98.544%. 